The present invention relates to microwave devices and components comprising dielectric substrates and conductors in the form of superconducting films. The tunability of such devices is obtained through varying the dielectric constant of the dielectric material. Examples of devices are for example tunable resonators, tunable filters, tunable cavities etc. Microwave devices or components are important for example within microwave communication, radar systems and cellular communication systems. Of course there are also a number of other fields of application.
The use of microwave devices is known in the art. In xe2x80x9cHigh Temperature Superconducting microwave circuitsxe2x80x9d by Z-Y Shen, Artech House 1994, dielectric resonators are discussed which are based on TE011 delta modes. A dielectric resonator is clamped between thin High Temperature Superconducting films (HTS) which are deposited on separate substrates and thus not directly on the dielectric. These resonators fulfill the requirements as to cellular communication losses and power handlings at about 1-2 GHz. It is however inconvenient that the dimensions of the HTS films and the dielectric substrates at these frequencies (e.g. 1-2 GHz) are large and moreover the devices are expensive to fabricate. Furthermore they can only be mechanically tuned which in turn makes the devices (e.g. filters) bulky and introduce complex problems in connection with vibrations or microphonics. WO 94/13028 shows integrated devices of ferroelectric and HTS films. Thin epitaxial ferroelectric films are used. Such films have a comparatively small dielectric constant and the tuning range is also limited and the microwave losses are high. Furthermore there is a highly non-linear current density in thin HTS film coplanar waveguides and microstrips. This results from the high current density at the edges of the strips, D. M. Sheen et al, IEEE Trans. on Appl. Superc. 1991, Vol. 1, No. 2, pp. 108-115. The applicability of these integrated HTS/ferroelectric thin film devices is therefore limited and they are not suitable as for example low-loss narrow-band tunable filters.
Generally tunable filters are important components within microwave communication and radar systems as discussed above. Filters for cellular communication systems for example, which may operate at about 1-2 GHz occupy a considerable part of the volume of the base stations, and often they even constitute the largest part of a base station. The filters are furthermore responsible for a high power consumption and considerable losses in a base station. Therefore tunable low loss filters having high power handling capabilities are highly desirable. They are also very attractive for future broad band cellular systems. Today mechanically tuned filters are used. They have dielectrically loaded volume resonators having dielectric constants of about 30-40. Even if these devices could be improved if materials were found having still higher dielectric constants and lower losses, they would still be too large, too slow and involve losses that are too high. For future high speed cellular communication systems they would still leave a lot to be desired.
In U.S. Pat. No. 5,179,074 waveguide cavities wherein either part of or all of the cavity is made of superconducting material are shown. Volume cavities with dielectric resonators have high Q-values (quality factor) and they also have high power handling capabilities. They are widely used in for example base stations of mobile communications systems. The cavities as disclosed in the above mentioned US patent have been reduced in size and moreover the losses have been reduced. However, they are mechanically tuned and the size and the losses are still too high. WO 94/13028 also shows a number of tunable microwave devices incorporating high temperature superconducting films. However, also in this case thin ferroelectric films are used as already discussed above, and the size is not as small as needed and the losses are too high. Furthermore, the tuning range is limited.
xe2x80x9c1 GHz tunable resonator on bulk single crystal SrTiO plated with YBaCuO films.xe2x80x9d by O. G. Vendik et al, Electronics Letters, Vol. 31, No. 8, April 1995 shows a tunable resonator on bulk single crystal SrTiO3 plated with YBCO films. This device however suffers from the drawbacks of not being usable above Tc (the critical temperature for superconductivity). This means for example that no signals could pass if the temperature would be above Tc which may have serious consequences in some cases. These devices cannot be used unless in a superconducting state.
Furthermore the superconducting films are very sensitive and since they are in no way protected this could have serious consequences as well. In general, in the technical field, only dielectrics e.g. photoresist have been used to protect superconducting films.
Thus tunable microwave devices are needed which can be kept small, operate at high speed and which do not involve high losses. Devices are also needed which can be tuned over a wide range and which do not require mechanical tuning. Devices are needed which have a high dielectric constant particularly at cryogenic temperatures and particularly devices are needed which fulfil the abovementioned needs in the frequency band of 1-2 GHz, but of course also in other frequency bands. Still further devices are needed which can operate in superconducting as well as in non-superconducting states. Devices are also needed wherein the superconducting films are less exposed. Particularly devices are needed which can be electrically tuned and reduced in size at a high level of microwave power.
Therefore a device is provided which comprises a substrate of a dielectric material with a variable dielectric constant. At least one superconducting film is arranged on parts of the dielectric substrate which comprises a non-linear dielectric bulk material. The substrate comprises a single crystal bulk material and the superconducting film or films comprise high temperature superconducting films. A normal conducting layer is arranged on one or both sides of the superconducting film(s) which is/are opposite to the dielectric substrate. The tuning is provided through producing a change in the dielectric constant of the dielectric material and this may particularly be carried out via external means and particularly the electrical dependence of the dielectric constant used for example for voltage control or also the temperature dependence of the dielectric constant can be used for controlling purposes. Particularly, an external DC bias voltage can be applied to the superconducting film. Alternatively a current can be fed to the films but it is also possible to use a heating arrangement connected to the superconducting film or films and in this way change the electric constant of the dielectric material. Bulk single crystal dielectrics particularly bulk ferroelectric crystals, have a high dielectric constant which can be above for example 2000 at temperatures below 100xc2x0 K, in the case of high temperature superconducting films below Tc, which is the transition temperature below which the material is superconducting. Krupka et al in IEEE MTT, 1994, Vol. 42, No. 10, p. 1886 states that bulk single crystal ferroelectrics such as SrTiO3 have small dielectric losses such as 2.6xc3x9710xe2x88x924 at 77xc2x0 K and 2 GHz and very high dielectric constants at cryogenic temperatures.
However, according to WO 94/13028 and xe2x80x9cA High Temperature Superconducting Phase Shifterxe2x80x9d by C. M. Jacobson et. al in Microwave Journal Vol. 5, No. 4, December 1992 pp 72-78 states that the electrical variation to change the dielectric constant of bulk material is small and thus far from satisfactory. Moreover, microwave integrated circuit devices are exclusively made by thin film dielectrics which according to the known documents is necessary.
The dimensions of the devices according to the invention can be very small, such as for example smaller than one centimeter at frequencies of about 1-2 GHz and still the total losses are low. This however merely relates to examples and the invention is of course not limited thereto.
Particularly the superconducting film arrangement and the dielectric substrate are arranged so that a resonator is formed and the superconducting film(s) may be arranged on at least two surfaces of the dielectric substrate. According to different embodiments the superconducting films may be arranged directly on the dielectric substrate or a thin buffer layer may be arranged between the superconducting films and the dielectric substrate. One aspect of the invention relates to the form of the parallel plate resonator wherein the dielectric substrate may comprise a resonator disc. More particularly at least one superconducting film (and normal conducting film arranged thereon) may have an area which is smaller, e.g., particularly somewhat smaller, than the corresponding area of the dielectric substrate on which it is arranged in order to provide coupling between degenerate modes thus providing a dual mode operation resonator. Even more particularly, in one aspect of the invention, it provides a two-pole tunable passband filter (or a multi-pole tunable filter). Means may be provided for controlling the coupling between the two or more degenerate modes.
According to still another aspect of the invention it is aimed at providing a tunable cavity. One or more resonators are then enclosed in a cavity comprising superconducting material or non-superconducting material. In the case of non-superconducting material, it may particularly be covered on the inside with a thin superconducting film. The cavity, still more particularly, comprises a below cut-off frequency waveguide. The device comprises coupling means for coupling micro-wave signals in and out of the device. These can be of different kinds as will be further described in the detailed description of the invention.
Moreover, in a particular embodiment of the invention second tuning means may be provided for fine-tuning or calibrating of the resonance frequency of the dielectric substrate of the resonator. These means may comprise a mechanically adjustable arrangement and can for example also comprise thermal adjusting means etc.
In a particular embodiment a cavity as referred to above may comprise two or more separate cavities each comprising at least one resonator. These resonators are connected to each other via interconnecting means and form a dual mode or a multi-mode resonator.
One example on a dielectric substrate is a material comprising SrTiO3 and the superconducting films may be so called YBCO-films (YBaCuO). The invention is applicable to a number of different devices such as tunable microwave resonators, filters, cavities etc. Particular embodiments relate to tunable passband filters, two three- or four-pole tunable filters etc. Other devices are phase shifters, delay lines, oscillators, antennas, matching networks, etc.
Tunable microwave integrated circuits are described in the copending patent application xe2x80x9cArrangement and method relating to tunable devicesxe2x80x9d filed at the same time by the same applicant, published as WO 96/42117 and which is incorporated herein by reference.